The present invention relates to the art of valveless positive displacement piston, metering pumps, and, in particular, to improvements which significantly enhance the accuracy of fluid delivery over the entire range of operation of such pumps
It has been known in the art of valveless positive displacement piston pumps to provide a reversible pumping function and controllable variable displacement by simple variation of the angle between two segments of the pump drive-axis. For example, in U.S. Pat. No. 3,168,872 and U.S. Pat. No. 4,008,003, both to Pinkerton, the contents of which are incorporated herein by reference, a valveless, variable, reversible pump is disclosed including a ducted piston which reciprocates and rotates synchronously in a bi-ported cylinder which is closed at one end to form a cylinder head chamber. The piston duct is arranged in the piston to provide a fluid transfer conduit in combination with the wall of the cylinder which is alternately in fluid communication with each of the ports such that one port is in communication with the cylinder head chamber on the down stroke of the piston and the other port is in communication with the cylinder head chamber on the up stroke. Reversal of the duct relationship to the ports results in reversal in direction of fluid flow.
In a typical pump of this type, to actuate the piston and effect the appropriate pump action, the piston assembly is coupled with the output of a drive shaft through an off-axis yoke assembly. The piston includes at its outer end a laterally extending arm which is slidably mounted in a spherical bearing member of the yoke assembly, whereby a single point universal joint is provided. The biported cylinder, which receives the piston, is mounted for articulation around a single central axis which is perpendicular to the axis of rotation of the yoke assembly. Thus when the axis of rotation of the yoke assembly (the drive axis) and that of the piston are substantially coaxial, the piston does not reciprocate in the cylinder during rotation of the yoke, and no pumping action takes place. However, when the cylinder axis - and thus the piston axis--is articulated (relative to the axis of the yoke) at the perpendicular axis, reciprocation occurs. The direction of deflection (to right or left) determines the direction of fluid feed through the pump chamber and the degree of angular movement determines the amplitude of piston stroke and, consequently, its displacement for each rotation of the drive motor shaft.
Inasmuch as diameter of the cylinder, the length of the piston stroke, and the stroke repetition rate are all determinable, the rate of fluid flow should, likewise, be dependably determinable. Surprisingly, however, dependable fluid flow control is not always possible, since unpredictable fluid inconsistencies can occur as a result of, for example, entrained or dissolved gases in the liquid stream which can grossly distort effective displacement values. This is particularly true in the low-flow portion of the flow rate range of such pumps because at low-flow settings they exhibit larger cylinder chamber dead-volume (a prime source of random bubbles) than at high flow settings. It will be seen therefore that since a large chamber dead-volume (low flow rate) poses a greater chance of bubbles lodging and flexing in the cylinder head chamber than a small volume (large flow rate), pumps of this type are often unsuitable for applications wherein accurate fluid delivery in the lower 15% of the possible flow rate range is required.
In view of the increasing demand for accurately adjustable rate flow pumps and the broadening scope of applications for them, a need exists to provide pumps that can be readily utilized for fluid delivery over an increased portion of the possible range of adjustment. Thus, it is an object of the present invention to provide a controllably variable and reversible positive displacement metering pump with a chamber dead-volume that may be minimized and remain constant in volume through the entire adjustment range of the pump whereby the accuracy of fluid delivery is significantly enhanced, even in the low volume portion of its operating range.
It is a further object of the invention to provide enhanced fluid delivery accuracy throughout the operating range of such pumps without modification of the basic pump and drive linkage design.
Another object of the present invention is to provide increased accuracy of fluid delivery over the full range of operation of such pumps utilizing the same method of determining direction of flow and adjustment of fluid delivery.
Other and further objects and advantages will become apparent from the following disclosure which is to be taken in conjunction with the accompanying drawings illustrating preferred as well as exemplary embodiments of the invention.